1. Field of the Invention
The present invention relates generally to an automatic sound volume controlling device for use in automotive vehicles in which sound volume is automatically controlled depending upon running-noises of the vehicle detected within the vehicle. More particularly, the invention relates to an improvement of a time constant circuit employed in such a device serving to delay the change of sound volume when the level of the detected running-noise changes abruptly.
2. Background Art
A prior art device is shown in FIGS. 2 and 3. In FIG. 2, a microphone 1 is disposed in a relevant position inside the vehicle. An amplifier 2 amplifies sound picked up by the microphone 1. A low-pass filter 3 allows to pass only low frequency components contained in the output of the amplifier 2. A detection circuit 4 detects noise components from the output of the low-pass filter 3. A rectifying circuit 5 rectifies the detection output supplied from the detection circuit 4. A logarithmic compression circuit 6 subjects the rectified d.c. output from the rectifying circuit 5 to logarithmic compression. A time constant circuit 7 imparts a long time constant upon the output of the logarithmic compression circuit 6 sufficient to extend its output. The output of the time constant circuit 7 is supplied to an electronic volume control 8, including, for instance, an A/D converter, etc. to thereby control the sound volume. The electronic volume control 8 controls the amplification of an audio signal which is then projected from a loudspeaker 9 with the desired sound volume.
Shown in FIG. 3 is a circuit diagram illustrating the time constant circuit 7, which is comprised of an operational amplifier 7a, a time constant sub-circuit including a resistor R.sub.1 and a capacitor C.sub.1, and a feed-back resistor R.sub.2.
In operation, the sound picked up by the microphone 1 is amplified by the amplifier 2 and only the running-noise of the vehicle is allowed to pass by the low-pass filter 3. Then, the running-noise is detected by the detection circuit 4 and is rectified by the rectifying circuit 5 to provide a d.c. signal. The d.c. signal outputted from the rectifying circuit 5 is applied to the logarithmic compression circuit 6, from which obtained is a logarithmically changing d.c. signal. The thus obtained d.c. signal is introduced into the time constant circuit 7 in which the time constant has been predetermined so that the output therefrom is not abruptly varied, that is, the output is made to vary smoothly when the level of the noise increases or decreases.
Between the output terminal of the operational amplifier 7a and its inverting input terminal, the capacitor C.sub.1 is connected to thereby shift a frequency response curve of the operational amplifier 7a to a lower frequency side so as to lower the level of the frequency response curve. As a result, the level of the sound volume is smoothly changed when the vehicle's speed changes. In FIG. 4, the change of the vehicle's speed is indicated by a solid line and the change of the level of the sound volume is indicated by a dotted line.
As is apparent from FIG. 4, when the running vehicle is decelerating and is finally stopped, the sound volume is still maintained at a large value despite the fact that the running-noise has decreased. This is caused by the large time constant of the time constant circuit 7. The duration when such a phenomenon occurs is indicated by character B in FIG. 4. Accordingly, the prior art device suffers from a drawback such that the large sound volume is maintained for a certain period of time thereby causing the listeners to feel distracted.